Active capillary and channel surfaces in separation devices can create problems in virtually any separation methodology, including chromatographic, electrophoretic and electroosmotic modalities. The charged surfaces of the capillaries and channels of these separation devices are particularly problematic in the separation of charged analytes such as proteins, peptides and nucleic acids. Charged biopolymer compounds are adsorbed onto the walls of the separation device, creating artifacts such as peak tailing, loss of separation efficiency, poor analyte recovery and poor retention time reproducibility. The interaction of biopolymers with the surfaces of the device seems to be the main reason for the loss in separation efficiency compared to that predicted by theory. The adsorption is believed to be due to the electrostatic interactions between positively charged residues on the biopolymer and negatively charged groups resident on the surface of the separation device.
Silica-based capillaries utilized in capillary electrophoresis have been modified with a range of coatings intended to prevent the adsorption of charged analytes to the walls of the capillaries. Surface deactivation methods include surface derivatization with poly(ethyleneglycol) or poly(ethyleneimine). See, for example Huang et al., J. Microcol. September 4, 135-143 (1992); and Bruin et al., Journal of Chromatogr., 471, 429-436 (1989).
Poly(ethyleneglycol)-like epoxy polymers that are functionalized with pendent hydroxy groups have also been formed on the surface of a silica-based capillary. This surface was shown to impart resistance towards protein adsorption to the capillary surface. See, Towns et al., Journal of Chromatogr., 599, 227-237 (1992).
Capillary surfaces have also been functionalized with poly(ethyleneimine) by etching the surface of the silica capillary with a sodium hydroxide solution followed by treating the etched surface with a poly(ethyleneimine) solution. Under acidic conditions, the polyimine coatings bear a positive charge which makes them particularly suited for the separation of basic proteins, since, at acidic pH, both the capillary surface and the proteins bear a positive charge. See, Erim, et al., Journal of Chromatogr., 708, 356-361 (1995) and references therein.
Other chemical modifications of the capillary have also been employed, such as polyacrylamide (Hjerten, J. Chromatogr., 347, 191 (1985)), glycol groups (Jorgenson, Trends Anal. Chem. 3, 51 (1984)), polysiloxanes and a glyceroglycidoxypropyl coating (McCormick, Anal. Chem., 60, 2322 (1998)).
For reproducible analyses and purifications, it is desirable that the coating on an electrophoretic or electroosmotic device remain substantially stable during the useful lifetime of the device. Additionally, to prevent the adsorption of charged analytes onto the surfaces of the device, hydrophilic polymers such as poly(ethyleneglycol) and hydroxyethylated poly(ethyleneimine) are particularly useful. Furthermore, for electroosmotic devices, it is desirable to have a coating bearing a charge that can be adjusted in magnitude by manipulating the conditions inside of the device (e.g., pH). An array of coatings which allowed the direction of electroosmotic flow to be chosen based on the choice of coating would also be of use. Surprisingly, the present invention provides coatings, methods and devices which possess these and other characteristics that will be apparent upon complete review of this disclosure.